Wavelength sweep control

ABSTRACT

Methods and apparatus for the active control of a wavelength-swept light source used to interrogate optical elements having characteristic wavelengths distributed across a wavelength range are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/686,941 filed Jan. 13, 2010, which is a continuation-in-partof co-pending U.S. patent application Ser. No. 12/541,770 filed Aug. 14,2009, which is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 11/755,131 filed May 30, 2007, which claims benefitof U.S. Provisional Patent Application Ser. No. 60/803,470, filed May30, 2006, all of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to determinationof a characteristic wavelength of an optical component and, moreparticularly, to techniques and apparatus for controlling the manner inwhich a spectral bandwidth is swept in an effort to determine thecharacteristic wavelength.

2. Description of the Related Art

Many optical components have a characteristic wavelength that may befound by interrogating the optical component with an optical sourcecapable of producing light at various wavelengths over a fixed range orbandwidth. For example, Bragg gratings (typically formed byphoto-induced periodic modulation of the refractive index of an opticalwaveguide core) are highly reflective to light having wavelengths withina narrow bandwidth centered at a wavelength generally referred to as theBragg wavelength. Because light having wavelengths outside this narrowbandwidth is passed without reflection, Bragg wavelengths can bedetermined by interrogating a Bragg grating with a light source sweptacross a bandwidth that includes the Bragg wavelength and monitoring thereflected optical power spectrum at a receiver unit. Because Braggwavelengths are dependent on physical parameters, such as temperatureand strain, Bragg gratings can be utilized in optical sensor systems tomeasure such parameters.

In these and a wide range of other types of optical systems, themeasurement of a characteristic wavelength of an optical component togreat accuracy (and/or with great repeatability) is important to systemperformance. Two significant parameters determining the error of anysuch measurement are the signal to noise ratio (SNR) and effectiveintegration time of the measuring system. SNR is dependent of manyfactors including received optical power, optical-source noise, andreceiver noise. The effective integration time is dependent on overallaveraging time and the proportion of that time which is producing usefulsignals at the receiver unit. Improving these two parameters can improvecharacteristic wavelength measurement repeatability and accuracy.

In a typical system, with a fixed spectral bandwidth sweep, a largepercentage of the interrogation time is spent covering wavelengths whereno useful signal is returned by the optical element under test. This maybe particularly true in the case where multiple elements (e.g., multipleBragg gratings disposed serially on a common fiber) are combined in acommonly used wavelength-division multiplexing (WDM) scheme. In thesearrangements, wavelength guard-bands are typically required between thespectral features of elements, for example, to ensure the elements havenon-overlapping spectral features over the entire expected measurementrange and even as some movement in the spectral features may be expectedover time. These guard-bands increase the total range of wavelengthsscanned, thereby increasing the amount of interrogation time spentcovering wavelengths that produce no useful signal.

Accordingly, techniques and systems that optimize the useful receivedsignal, reduce SNR, and reduce the total amount of interrogation timewould be desirable.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally provide methods andapparatus for interrogating sensors elements having characteristicwavelengths spread across a wavelength range.

One embodiment of the present invention is a method. The methodgenerally includes filtering light emitted from an amplified spontaneousemission (ASE) source, amplifying the filtered light, and interrogatingone or more optical elements with the amplified light to measure one ormore parameters.

Another embodiment of the present invention provides an apparatus forinterrogating one or more optical elements. The apparatus generallyincludes an ASE source for emitting light, a filter for filtering thelight emitted by the ASE source, and an amplifier for amplifying thefiltered light, such that the amplified light is used to interrogate theoptical elements.

Yet another embodiment of the present invention is a method. The methodgenerally includes providing light, wherein a wavelength of the light ischanged according to a sweep function; interrogating one or morereflective optical elements with the wavelength-swept light to producereflected optical signals; filtering the reflected optical signals,wherein a bandpass wavelength range is changed based on the sweepfunction to follow the changes in the light's wavelength; and receivingthe filtered, reflected optical signals for processing.

Yet another embodiment of the present invention provides an apparatus.The apparatus generally includes a light source configured to change awavelength of the light according to a sweep function; one or morereflective optical elements configured to receive the wavelength-sweptlight from the light source and to reflect portions of the light atcharacteristic wavelengths producing reflected optical signals; atunable bandpass filter configured to filter the reflected opticalsignals, wherein a bandpass wavelength range of the filter is changedbased on the sweep function to follow the changes in the light'swavelength; and a receiver for processing the filtered, reflectedoptical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A illustrates an exemplary transmissive optical sensor system withwavelength sweep control;

FIG. 1B illustrates an exemplary reflective optical sensor system withwavelength sweep control;

FIG. 2 illustrates an exemplary wavelength sweeping optical sourceutilizing a tunable filter;

FIG. 3 illustrates how sweep rates may be varied for differentwavelength regions of interest in accordance with embodiments of thepresent invention;

FIG. 4 illustrates how optical power may be varied for differentwavelength regions of interest in accordance with embodiments of thepresent invention;

FIG. 5 illustrates how wavelength features of interest may shift overtime and how sweep rates of corresponding wavelength regions may beadjusted accordingly;

FIG. 6 is a flow diagram of exemplary operations for varying wavelengthsweep parameters based on feedback from previous sweeps;

FIG. 7 is a flow diagram of exemplary operations for varying wavelengthsweep parameters of a current sweep based on feedback;

FIG. 8 is a flow diagram of exemplary operations for varying sweep ratesbased on specified sensor resolutions;

FIG. 9 is a flow diagram of exemplary operations for automaticallydiscovering a sensor topology during a sweep of a range of wavelengths;

FIG. 10 illustrates an exemplary wavelength sweeping optical sourceutilizing an amplified spontaneous emission (ASE) source, a tunablefilter, and an amplifier;

FIG. 11 illustrates an exemplary reflective optical sensor system withthe optical source of FIG. 10;

FIG. 12 illustrates an exemplary reflective optical sensor system withan exemplary ASE source and an exemplary optical amplifier;

FIGS. 13A-B illustrate exemplary reflective optical sensor systems forswept-wavelength interrogation with a tunable filter located between acirculator and a receiver in an effort to filter out reflected signalsthat do not arise from the wavelength and time slot of interest during asweep;

FIG. 13C illustrates the timing of sourcing an optical signal ofwavelength i during a sweep window into an optical waveguide having tworeflective sensor elements with a characteristic wavelength i andreceiving the reflected optical signals after filtering, in accordancewith FIG. 13A or 13B, for example;

FIG. 14 illustrates an exemplary reflective optical sensor system forswept-wavelength interrogation with a tunable filter located between thereflective sensor elements and a circulator in an effort to filter outreflected signals that do not arise from the wavelength and time slot ofinterest during a sweep before such signals reach the receiver;

FIGS. 15A-B illustrate exemplary reflective optical sensor systems forswept-wavelength interrogation with a tunable filter located in theeffective cavity of the light source in an effort to filter outreflected signals that do not arise from the wavelength and time slot ofinterest during a sweep;

FIG. 16A illustrates an exemplary reflective optical sensor system forswept-wavelength interrogation with a tunable filter located between thereflective sensor elements and a circulator in an effort to filter outreflected signals that do not arise from the wavelength and time slot ofinterest during a sweep; and

FIG. 16B illustrates an exemplary reflective optical sensor system forswept-wavelength interrogation with a tunable filter located in theeffective ring cavity of the light source in an effort to filter outreflected signals that do not arise from the wavelength and time slot ofinterest during a sweep.

DETAILED DESCRIPTION

Embodiments of the present invention provide for the active control of alight source used to interrogate optical elements having characteristicwavelengths distributed across a wavelength range.

For some embodiments, this active control may include varying sweeprates across different ranges. For example, a sweep rate may be reducedin ranges containing spectral features of interest, allowing moremeasurements, which may lead to increased resolution. On the other hand,the sweep rate may also be increased in order to skip, or otherwise moverapidly through, other ranges (e.g., ranges absent features of interestor ranges corresponding to measured parameters that do not require ashigh resolution as others or as frequent measurements). Further, forsome embodiments, particular ranges (sweep bands) may be adjusted, forexample, to follow features of interest as they shift (e.g., change inwavelength) over time.

Different embodiments of the present invention may utilize wavelengthsweep control described herein in systems utilizing transmissive orreflective type sensors. Further, embodiments of the present inventionmay be applied in a number of different sensing applications, including,but not limited to, industrial applications, downhole applications(e.g., in wellbore sensing applications), and subsea applications (e.g.,ocean bottom seismic sensing applications).

An Exemplary System

FIG. 1A illustrates an exemplary optical sensor system 100 utilizingwavelength sweep control in accordance with one embodiment of thepresent invention. As illustrated, the system 100 may include aswept-wavelength optical source 110, one or more transmissive opticalelements 120 having one or more spectral features of interest (e.g., acharacteristic wavelength), and a sweep control unit 140.

The swept-wavelength optical source 110 produces optical radiation atwavelengths and over wavelength ranges (bandwidths) under the control orinfluence of the sweep control unit 140. The elements 120 may beinterrogated with optical radiation from the optical source 110 that isswept across a spectral range including the spectral features ofinterest. The elements 120 may be sensitive to parameters (e.g.,temperatures, pressures and strain) that effect the attenuation ofparticular wavelengths of light transmitted through the elements 120 ina known manner.

As illustrated in FIG. 2, one embodiment of the optical source 110 mayinclude a broadband source 112 and a tunable filter 114 that may becontrolled by the sweep control unit 140. For example, the sweep controlunit 140 may control the tunable filter 114 to adjust a wavelength range(or band) to pass with little or no attenuation while blockingwavelengths outside the range. For other embodiments, the optical source110 may include a light source that can be controlled to generateoptical signals of different wavelengths, such as a tunable laser.

Referring back to FIG. 1A, a receiver 130 may include any suitablecombination of optical, opto-electronic, and electronic components toprocess light signals transmitted through the elements 120. Thus, thereceiver 130 may be able to generate information about the correspondingparameters, based on the spectral information extracted from thereceived light. The receiver 130 may include any suitable combination ofcomponents that converts optical signals to electrical signals,integrates, filters and produces characteristic wavelengthdeterminations. As an example, for one embodiment, the receiver mayinclude an optical PIN diode, transimpedance amplifier, analog filter,analog-to-digital converter, digital filter and processing unit (e.g.,an embedded processor, industrial or personal computer) for wavelengthdetermination.

As illustrated, the sweep control unit 140 may receive, as input, one ormore signals from one or more points in the receiver 130 and, inresponse, may output signals that influence the sweep of the opticalsource 110. Examples of typical parameters that the sweep control unitmay influence include, but are not limited to, source wavelength, sourcewavelength sweep range, sweep rate, and/or source optical output power.These influences may include discontinuous or continuous changes in suchparameters, for example, multiple sweep bands (FIG. 3). The sweepcontrol unit signals can influence a sweep as it is in progress and/orinfluence future sweeps, as will be described in greater detail below.

The sweep control unit 140 may be implemented using any suitableprocessing logic, such as an embedded controller, a programmable logiccontroller (PLC) or personal computer (PC). While shown as a separatecomponent in the Figures, for some embodiments, the sweep control unit140 may be integrated into, or be an integral function of the receiver130, source 110, and/or both.

As illustrated in FIG. 1B, similar techniques may be applied to a systemutilizing reflective sensor elements 122, such as Bragg gratings, withthe spectral feature of the light reflected dependent upon a sensedparameter. Each Bragg grating 122 may be interrogated by sweeping acrossa corresponding wavelength range chosen to contain the characteristicwavelength λ, accounting for the maximum deviations in centerwavelengths (areas of peak reflection) expected over the entire range ofmeasured parameters and over time. During this interrogation, responsesignals are monitored by the receiver 130 in order to makecharacteristic wavelength determinations.

Interrogating optical signals from the source 110 may be directed to thegratings 122 via a bi-direction coupler 124 that also directs reflectedresponse signals to the receiver 130. A splitter 126 may also direct aportion of the interrogating optical signals to a reference element 116,allowing the receiver 130 to monitor optical signals produced by theoptical source 110 (e.g., the actual wavelength and power).

As previously described, wavelength division multiplexed (WDM) systems,such as the system 100 typically have deadbands between sensorwavelengths, to ensure non-overlapping characteristic wavelengths. Inconventional systems, these deadbands add to the total swept wavelengthrange, thereby increasing overall interrogation time and decreasing thepercentage of this time a useful response signal is produced. However,embodiments of the present invention may increase the percentage of timespent producing useful response signals by skipping these deadbands orat least increasing the sweep rate to rapidly sweep through them.

Varying Sweep Rates

FIG. 3 illustrates an exemplary spectral response for a system (power ofreceived response signals versus wavelength), with multiple swept ranges310 containing spectral features of interest 312. As illustrated,regions of interest may be scanned with a first (relatively slow) scanrate, while deadbands 320 may be scanned with a second (relativelyfaster) scan rate or skipped altogether. For some embodiments, forexample, due to limited response time of the source 110 (e.g., due tophysical, mechanical, or electrical limitations), it may not be possibleto entirely skip a wavelength range and therefore deadbands may be sweptwith increased sweep rate (relative to the ranges of interest 310).

In either case, controlling the sweep rate in this manner may increasethe useful optical energy received from the optical elements in a giveninterrogation time. As a result, overall interrogation time may bereduced relative to conventional systems or, alternatively, moremeasurements may be taken in the same interrogation time, allowing anincreased “focus” on ranges of interest, which may increase accuracy.

Different sweep rates may also be utilized for different ranges ofinterest, to interrogate different sensors at different rates, which mayprovide a great deal of flexibility in overall system design. Forexample, a first sensor (e.g., having a first characteristic wavelengthλ1) may be interrogated using a lower sweep rate than that used tointerrogate a second sensor (λ2). As a result, more measurements may betaken for the first sensor, which may be lead to higher accuracymeasurements, while the second sensor may be used for more coarsemeasurements. Using this approach, some sensors may be designated as“high resolution” sensors and interrogated with lower sweep rates(sampled more often) than other sensors.

At a different point in time, it may become desirable to take higheraccuracy measurements of the second sensor. Therefore, the sweep ratesof different sensors may be changed from one sweep to the next. Forexample, for some applications, it may only be necessary to take highlyaccurate measurements of certain parameters in certain situations (e.g.,when the parameter is changing rapidly, or has reached a particularthreshold value). In some instances, high accuracy measurements (lowsweep rate) of a particular parameter may only be made when a coarsemeasurement of the same parameter (taken in a current or previous sweep)indicates a particular value or range.

As illustrated in FIG. 4, for some embodiments, the optical power ofinterrogating light signals may also be varied for different sweptranges (as an alternative to, or in conjunction with, varying sweeprates). For example, optical power may be decreased when sweeping acrossdead ranges. This approach may allow optical power to be conserved. Forsome embodiments, reduced optical power may be used to scan particularswept ranges, until a particular threshold level of optical responsesignal is received.

Changes in the received power from the optical element (or opticalsystem) could also be compensated for, by adjusting the source outputpower for example. As will be described in greater detail below, withreference to FIG. 9, monitoring response signals while quickly sweepingand/or interrogating with lowered optical power over particular sweptranges may be performed as part of a process to automatically “discover”a particular sensor topology.

Adjusting Ranges of Interest

Embodiments of the present invention may also allow for only a limitedband of wavelengths directly surrounding particular spectral features ofinterest to be swept by the source. The wavelength sweep control unitmay continuously adjust the swept bands/ranges to track these features,should they change in wavelength over time.

For example, as illustrated in FIG. 5, the characteristic wavelength ofa first sensor (λ1) may change over time, such that the region ofinterest, defined by the expected deviation in wavelength of the sensor,may shift over time. A previous region of interest is shown as a dashedline, while the new region of interest is shown as a solid line. In theillustrated example, a positive shift for λ1 is shown. As illustrated inthe upper graph of FIG. 5, in response to this shift, the wavelengthsweep control 140 may adjust the corresponding swept range (swept with arelatively low sweep rate and/or a relatively high optical power) for λ1to compensate for the shift. As illustrated, the characteristicfrequency for a second sensor (λ2) may shift in the opposite direction,which may cause the wavelength sweep control 140 to adjust thecorresponding swept range accordingly.

FIG. 6 is a flow diagram of exemplary operations that may be performed,for example, by the wavelength sweep control 140 to vary wavelengthsweep parameters based on feedback from previous sweeps. At step 602, asweep begins, for example by interrogating optical elements with lightsignals having a wavelength at a low end of a total range to be swept.As described above, the total range to be swept may be divided intoranges (e.g., ranges of interest and deadbands).

At step 604, a loop of operations is entered, to be performed for eachrange. At step 606, a determination is made as to if a current rangecontains a spectral feature of interest. If the current range does notcontain a spectral feature of interest, the range can be skipped or, atleast, scanned rapidly, at step 612. If the current range contains aspectral feature of interest, wavelengths in the range may be swept at aspecified (relatively slow) sweep rate, at step 608. At step 610, thereceived power (response signal) may be recorded for later use.

The operations may be repeated (e.g., slowly sweeping ranges of interestand rapidly sweeping deadbands), until all ranges have been swept. Atstep 614, the swept ranges may be adjusted based on the recordedreceived power, for example, as described above with reference to FIG.5. These adjusted swept ranges may then be used in a subsequent sweep.In this manner, the wavelength sweep control 140 may continuously adjustsweep parameters to compensate for changing sensor characteristics.

FIG. 7 is a flow diagram of exemplary operations for varying wavelengthsweep parameters of a current sweep based on feedback. The operationsshown in FIG. 7 may be performed to sweep without using predefined sweepranges, for example, by sweeping rapidly until some level of responsesignal is detected indicating a sensor region of interest has beenreached. As an alternative, the operations of FIG. 7 may be performedwith predefined sweep ranges, for example, in an effort to detectspectral information occurring in what was thought to be a deadband.

At step 702, a sweep begins. At step 706, the optical response ismonitored. As long as the response does not exceed a predeterminedthreshold, as determined at step 708, the wavelength is adjustedrapidly. Once the response does exceed the predetermined threshold, thewavelength is adjusted slowly. These operations may repeat, until theend of a swept range has been reached, as determined at step 704. Thus,these operations may allow regions that contain no spectral feature ofinterest (as evidenced by a lack of response signal) to be quicklyscanned.

FIG. 8 is a flow diagram of exemplary operations for varying sweep ratesbased on specified sensor resolutions. As previously described, somesensors may be identified as high resolution sensors that may be scannedslower (allowing more samples to be taken) or that may be scanned withinterrogating signals having higher optical power. Other sensors,identified as low-resolution sensors may be scanned more rapidly(although not as quickly as a deadband) or that may be scanned withinterrogating signals having relatively lower optical power.

At step 802, a sweep begins and, at step 804, a loop of operations isentered, to be performed for each range. At step 806, a determination ismade as to if a current range contains a characteristic wavelength of acorresponding sensor. If the current range does not contain a sensorwavelength, the range can be skipped or, at least, scanned rapidly, atstep 812. If the current range contains a sensor wavelength, adetermination is made, at step 808, as to whether the correspondingsensor is a high or low-resolution sensor.

If the sensor is a low-resolution sensor, the range may be scanned witha relatively fast sweep range (but slower than that used to sweep adeadband), at step 810. If the sensor is a high-resolution sensor, therange may be scanned with a relatively slow sweep range, at step 814.The operations may be repeated until all ranges have been swept.

FIG. 9 is a flow diagram of exemplary operations for automaticallydiscovering a sensor topology during a sweep of a range of wavelengths.The operations may be performed, for example, as an initial operation todetermine the types of sensors that are present in an optical systemwithout requiring field personnel to enter corresponding data manually.In some cases, sensor vendors may sell sensors with known characteristicwavelengths (or wavelength ranges), allowing corresponding data to bepre-stored in the system. In such cases, if the characteristicwavelengths are automatically detected during a sweep, it may be asimple matter of looking up the actual device characteristics, such asthe response changes in wavelength as a function of a correspondingmeasurand (e.g., pressure, temperature, strain, and the like).

At step 902, a sweep of a wavelength range begins. At step 904, adetermination is made as to if the end of the range has been reached. Ifnot, the optical response is monitored (or continues to be monitored),at step 906. At step 908, if the monitored response does not exceed apredetermined threshold (e.g., indicating the absence of acharacteristic wavelength at or near the current swept wavelength), thewavelength may be adjusted rapidly, at step 910.

On the other hand, if the monitored threshold exceeds a predeterminedthreshold (e.g., indicating a characteristic wavelength at or near thecurrent swept wavelength), the start of a sensor range may be recorded,at step 912. Because the current wavelength may be at or near acharacteristic sensor wavelength, the wavelength may be adjusted slowly,at step 914, while continuing to monitor the optical response, at step916. The sensor range may include all wavelengths for which themonitored response remains above the predetermined threshold. If themonitored response falls below the predetermined threshold (in somecases allowing for some amount of hysteresis), as determined at step918, the end of the sensor range may be recorded, at step 920. Theoperations may be repeated until the entire range has been swept.

Those skilled in the art will also recognize that different aspectsdescribed herein may be combined, for some embodiments. As an example,for some embodiments, wavelength sweep control logic may be configuredto perform different combinations of operations shown in the flowdiagrams described above, to provide different combinations of features.

Amplifier Configuration for a Bragg Grating Interrogator

For some embodiments, an amplified spontaneous emission (ASE) source maybe utilized as the optical source 110 for interrogating the opticalelements. Spontaneous emission can occur in an optical fiber whenelectrons in an upper energy level decay to a lower energy level,spontaneously emitting photons in all directions. Some of these photonsare emitted in a direction falling within the numerical aperture of thefiber such that these particular photons are captured and guided by thefiber. In a doped optical fiber, the captured photons from the initialspontaneous emission may then interact with dopant ions and consequentlybe amplified by stimulated emission, hence the term “amplifiedspontaneous emission.” Accordingly, ASE may be considered as light,produced by spontaneous emission, that has been optically amplified bythe process of stimulated emission in a gain medium.

However, the spectral power density of typical ASE sources is lowcompared to ordinary laser output power densities. In swept-wavelengthgrating interrogation systems, such low ASE source output spectral powerdensity can strain the optical power budget, thereby limiting themaximum sensor reach.

Historically, this problem has been addressed by a number of approacheshaving varying success and a number of disadvantages. One approach hasbeen to work within the constraints of the resulting optical powerbudget by employing highly sensitive receivers. However, such receiverscan be expensive, and the improvements are limited. Another approach hasbeen to simply try to increase the output power of the ASE source.

To overcome these problems, FIG. 10 illustrates an exemplary wavelengthsweeping optical source utilizing an ASE source 200, a tunable filter114, and an amplifier 210. As described above, ASE is produced when again medium is stimulated (e.g., pumped) to produce a populationinversion. The ASE source 200 may comprise an optical fiber doped withdopant ions and having a length of several meters as the laser gainmedium. For example, the core of a silica fiber may be doped withtrivalent erbium ions (Er⁺³) to fabricate an erbium-doped fiber. Pumpingmay be achieved with electrical currents (e.g., produced bysemiconductors, or by gases via high-voltage discharges) or with light,generated by discharge lamps or by other lasers (e.g., semiconductorlasers). A laser used to pump a doped fiber is known as a pump laser.

The tunable filter 114 may function as described above to produce anarrow band (i.e., range) of wavelengths from the broadband ASE source200. The narrow wavelength band passing through with little or noattenuation may be adjusted as the filter 114 is tuned. For someembodiments, the tunable filter 114 may be controlled by the sweepcontrol unit 140 as described above.

The amplifier 210 may boost the narrow band swept-wavelength signal fromthe tunable filter 114 and output the amplified signal for interrogationof the optical elements, such as the transmissive optical elements 120or the reflective sensor elements 122 described above. For example, theamplifier 210 may provide a gain of 30 dB, such that 20 to 50 μW may beamplified to 20 to 50 mW. Using the amplifier 210 after the tunablefilter 114 may provide for a substantial increase in the output signallevel of the optical source, independent of the constraint to preventlasing in the ASE source 200.

The amplifier 210 may comprise an optical amplifier, which amplifies alight signal directly. For some embodiments, the optical amplifier maycomprise a doped fiber amplifier (DFA). A DFA is an optical amplifierthat uses a doped optical fiber as a gain medium to amplify an opticalsignal. In a typical DFA, the optical signal to be amplified and lightfrom a pump laser (pump light) are multiplexed into the doped fiber, andthe signal is amplified through interaction with the dopant ions. Morespecifically, the pump light excites the dopant ions to higher energylevels (orbits), and the input optical signal stimulates the exciteddopant ions to release excess energy as photons in phase and at the samewavelength as the input signal. The doped fiber may comprise erbium ionsto produce an erbium-doped fiber (EDF), although dopant ions of thulium,praseodymium, or ytterbium have also been implemented.

For other embodiments, the optical amplifier may comprise asemiconductor optical amplifier (SOA). An SOA is typically made fromgroup III-V compound semiconductors, such as GaAs/AlGaAs, InP/InGaAs,InP/InGaAsP, and InP/InAlGaAs. Although an SOA is generally lessexpensive than a DFA and can be integrated with semiconductor lasers,current SOAs have higher noise, lower gain, moderate polarizationdependence, and high nonlinearity with fast transient time. However, anSOA may provide for gain in different wavelength regions than a DFA.

By amplifying the narrow band swept-wavelength emission from the ASEsource 200 as illustrated in FIG. 10, the limitations on the opticalpower budget may be resolved, and the ASE source 200 may be suitable foruse in a swept-wavelength optical sensor system.

FIG. 11 illustrates an exemplary reflective optical sensor system 1100employing the optical source 110 of FIG. 10. This system 1100 is similarto the reflective optical sensor system 100 of FIG. 1B and includes thebi-directional coupler 124, the reflective sensor elements 122, and thereceiver 130. Interrogating optical signals produced by tuning andamplifying light emitted from the ASE source 200 may be directed to thereflective sensor elements 122 (e.g., fiber Bragg gratings, or FBGs) viathe bi-directional coupler 124. The coupler 124 may also direct responsesignals reflected from the reflective sensor elements 122 to thereceiver 130 for optical detection and signal processing. For someembodiments, the coupler 124 may be replaced with an optical circulator.

Some embodiments may include an optional reference receiver 220 in aneffort to monitor optical signals produced by or internal to the opticalsource 110. The reference receiver 220 may monitor optical signalsbefore and/or after the amplifier 210 as shown. The reference receiver220 may incorporate a reference element 116 as described above. Theoptical signals may be directed to the reference receiver 220 via asplitter (not shown), similar to the splitter 126 of FIG. 1B. Thereference receiver 220 may be independent from the receiver 130 or maybe incorporated (at least partially) into the receiver 130.

For some embodiments, the reflective sensor elements 122 may comprisecane-based gratings, where the gratings are inscribed in a largediameter waveguide (referred to as a “cane waveguide”) rather than in anoptical fiber. Cane waveguides have a core and a cladding just as dostandard fibers. In fact, the core of a single mode cane is generallythe same diameter as the core of a single mode standard fiber, typically7 to 12 μm (microns). However, cane is thicker and sturdier than fiberbecause of the substantial amount of cladding. While a standard fiberhas a diameter of 125 μm, cane typically ranges from 0.3 mm to about 4mm, the great bulk of which constitutes cladding. The cane's relativelythick cladding provides significant mechanical benefits over fiber.Furthermore, a cane does not require a protective buffer layer and,thus, eliminates manufacturing complexity.

FIG. 12 illustrates an exemplary reflective optical sensor system 1200with an exemplary ASE source and an exemplary optical amplifier. The ASEsource may comprise an erbium-doped fiber (EDF) 1202, awavelength-division multiplexer (WDM) 1204 with an optical isolator1206, and a pump laser 1208 as shown. Decay of electrons in the upperenergy level may cause spontaneous emission of photons within the EDF1202. Pump light from the pump laser 1208 may be multiplexed into theEDF 1202 via the WDM 1204 to excite erbium ions to higher energy levels(orbits) in the EDF. The spontaneously emitted photons may stimulate theexcited erbium ions to release excess energy as photons, such that theEDF 1202, the WDM 1204, and the pump laser 1208 function as an ASEsource.

The ASE source may be coupled to the tunable filter 114 via the opticalisolator 1206, which directs the light emitted by the ASE source to thefilter and blocks light reflected from the filter. The tunable filter114, in turn, may be coupled to the optical amplifier via anotheroptical isolator 1210. This optical isolator 1210 may direct filteredlight to the optical amplifier and block backwards scattered ASE lightfrom the optical amplifier.

The optical amplifier may comprise an erbium-doped fiber (EDF) 1212, awavelength-division multiplexer (WDM) 1214 with an optical isolator1216, and a pump laser. As depicted in FIG. 12, the ASE source and theoptical amplifier may share the same pump laser 1208, and a splitter1218 may be used to direct a portion of the pump light to the ASE sourceand the remaining portion to the optical amplifier. For someembodiments, the splitter 1218 may be a 90:10 splitter or an 80:20splitter. For example, the 90:10 splitter may direct 90% of the pumplight to the ASE source and 10% to the optical amplifier. Since theoptical signal input to the amplifier is low (e.g., 20 to 50 μW), only asmall amount of power may be required to pump the amplifier,substantially less power than is used for ASE. Consequently, the samepump laser 1208 may be used for pumping both the ASE source and theoptical amplifier.

In the amplifier of FIG. 12, the narrow wavelength range optical signalfrom the tunable filter 114 and pump light from the pump laser 1208 maybe multiplexed into the EDF 1212 via the WDM 1214 to excite erbium ionsto higher energy levels (orbits) in the EDF. The input optical signalmay stimulate the excited erbium ions to release excess energy asphotons, such that the EDF 1212, the WDM 1214, and the pump laser 1208function as an erbium-doped fiber amplifier (EDFA). The WDM 1214 may becoupled to the isolator 1216, such that the amplified optical signal maybe directed to the optical sensor elements, but the pump light isblocked.

The optical sensor system 1200 may include a splitter 1220 (e.g., a90:10 splitter) for directing a portion of the amplified optical signalto a comb filter 1222 and a reference receiver 1224. The comb filter1222 may produce a reference spectrum having spectrum peaks with aconstant, known frequency separation for use as an “optical ruler”during signal processing of the response signals reflected from thesensor elements 122.

A remaining portion of the amplified optical signal may be directed toan optical coupler 1226. The optical coupler 1226 may direct a portion(e.g., half) the amplified optical signal to the reflective sensorelements 122, and a remaining portion (e.g., the other half) to areference Bragg grating 1228. Light reflected by the reflective sensorelements or the Bragg grating 1228 may be passed back through theoptical coupler 1226 and directed to the sensor receiver 1230 forconversion to electrical signals (via a photodiode, for example) andfurther signal processing. In this manner, accurate sensor measurementsmay be performed for measuring parameters such as temperature, pressure,and/or strain.

Interrogating WDM/TDM Sensors Using FDML Techniques

As a practical matter, the number of optical elements (e.g., fiber Bragggratings) that can be used in a single interrogation system is limitedby the ability of the interrogating instrument to distinguish betweenthe optical elements. Wavelength division multiplexing (WDM) and timedivision multiplexing (TDM) have been used to distinguish betweengratings by wavelength and time, respectively. However, even moreoptical elements could be distinguished if both WDM and TDM were enabledon the same optical waveguide by the interrogating instrument.

Accordingly, some embodiments of the present invention provide foradding (or, in some cases, simply relocating) a swept-wavelength tunableoptical filter in front of the receiver of an interrogator system. Inthis manner, it is possible to filter out optical signals received fromthe optical elements that do not result from the wavelength and timeslot of interest during a wavelength sweep. Such embodiments provide forboth WDM and TDM of the optical elements, which enables many moreoptical elements to be distinguished and added to an optical waveguide,such as an optical fiber. Moreover, such embodiments have the addedbenefit of filtering out Rayleigh scattering and connectorback-reflections from the optical fiber transmitting the signals,thereby allowing ultra long reach sensing to be achieved.

For example, FIG. 13A illustrates an ultra long reach opticalinterrogator system 1300 using reflective sensor elements 122, such asfiber Bragg gratings (FBGs) written in an optical fiber 1301. Althoughtransmissive optical elements 120 may replace the reflective sensorelements 122 in some embodiments using both WDM and TDM, the remainderof the specification will refer to only reflective sensor elements forease of description.

The reflective sensor elements 122 may have various characteristicwavelengths λ₁ to λ_(N) for WDM, where N is the maximum number ofdiscrete characteristic wavelengths in an optical waveguide.Furthermore, characteristic wavelengths of the reflective sensorelements 122 may be repeated one or more times on the same optical fiber1301, thereby implicating TDM, as well. As illustrated in FIG. 13A, theoptical fiber may have at least two reflective elements with the samecharacteristic wavelength (e.g., λ_(2,a) and λ_(2,b) or λ_(N,a) andλ_(N,b)), which may be separated by a length of optical fiber. Becausethese reflective sensor elements 122 are positioned at differentlocations along the optical fiber 1301, the interrogator system may mostlikely be able to distinguish between them in time using TDM, eventhough these elements have the same characteristic wavelength. However,the various characteristic wavelengths need not be repeated the samenumber of times on the optical fiber or be arranged in any order (e.g.,ascending or descending wavelength). In fact, separating reflectivesensor elements 122 with closely-valued characteristic wavelengths alongthe optical fiber may offer better distinguishing ability, especially ininstances where the optical filter has a wide wavelength passband andthe characteristic wavelengths are close together (i.e., have smalldifferences in wavelength). Also, repeated characteristic wavelengths ofa reflective sensor element group need not follow the arrangement orderof another group.

In the interrogator system 1300 of FIG. 13A, the optical source110—which may include any of the embodiments described above—may becoupled to an optical circulator 1302 for delivering theswept-wavelength optical signal to the reflective sensor elements 122.Signals reflected from the sensor elements 122 may be directed by thecirculator 1302 to a tunable optical filter 114 and the receiver 130 forsignal processing. As the optical source 110 outputs differentwavelengths of light during a sweep, the tunable optical filter 114 mayhave a narrow bandpass wavelength range that filters out reflectedsignals that are not due to the sourced wavelength from reaching thereceiver 130. These blocked optical signals include reflections fromsensor elements having characteristic wavelengths different from thesourced wavelength during a particular period. Furthermore, the tunablewavelength passband of the optical filter 114 may be adjusted accordingto a sweep function to follow the changes in the wavelength emitted bythe swept-wavelength optical source 110. Because the filter's passbandmay be adjusted in time according to the sweep function, the tunablefilter 114 may also filter out reflected signals that do not result fromthe desired time slot from reaching the receiver 130. These blockedsignals may not only include back-reflections from, for example, opticalconnectors, circulators, couplers, and/or other reflective sensorelements, but also Rayleigh scattering within the optical fiber 1301.

The tunable optical filter 114 may be synchronized to sweep (i.e.,adjust the tunable passband) with a delay in relation to the sourcewavelength sweep. As illustrated in FIG. 13A, the delayed sweep signal1304 may link the wavelength sweep of the optical source 110 to thetuning of the filter 114. For some embodiments, delay may be constantand may be set approximately equal to the round trip travel time down tothe first reflective sensor element 122 and back again. For otherembodiments, the delayed sweep signal 1304 may be wavelength-dependentsuch that for a given wavelength i, the delay may be set approximatelyequal to the round trip travel time down to the first reflective sensorelement 122 having that particular characteristic wavelength (λ_(i)).Still other embodiments may not include a delay between the sourcedwavelength sweep and the tuning of the reflected-signal filter. Thedelay may also be changed over time in order to receive signals fromother reflective sensor elements located with different round triptravel times.

For some embodiments as illustrated in FIG. 13B, the wavelength-sweptoptical source 110 may be replaced by a broadband source 112 emittingbroadband light and a second tunable optical filter 114 for filteringthe broadband light to produce wavelength-swept light. For suchembodiments, wavelength sweep control of the source filter after thebroadband source 112 and the passband adjustment of the reflected-signalfilter before the receiver 130 may be managed by a sweep control unit140, similar to those described above. The sweep control unit 140 mayprovide no delay, a constant delay, a time-dependent delay, or awavelength-dependent delay as described above between the source filterand the reflected-signal filter.

FIG. 13C illustrates the wavelength-swept interrogation timing, inaccordance with the embodiments of FIG. 13A or 13B, for example, for anoptical waveguide having two reflective sensor elements 122 with acharacteristic wavelength i. The optical waveguide may comprise morethan two reflective sensor elements, but two elements will suffice forthis explanation. The first reflective sensor element withcharacteristic wavelength i (λ_(i,a)) is positioned at a first length l₁down the optical waveguide away from the source 110 and the receiver130, assuming that the source and the receiver are the same distanceaway from the first reflective sensor element. The second reflectivesensor element with characteristic wavelength i (λ_(i,b)) is positionedat a second length l₂ down the optical waveguide away from the source110 and the receiver 130.

At a certain time t₀, the wavelength-swept optical source 110 may beginemitting light at (or in a range containing) the particularcharacteristic wavelength i during the sweep. The source may emit atwavelength i for a gating period of time τ_(gate) gate defining a sweepwindow 1310. For simplicity of illustration and explanation, the otherwavelengths or wavelength ranges emitted during the sweep at prior orsubsequent times are not shown in FIG. 13C. The sourced light atwavelength i may travel down to the reflective sensor elements 122, afirst portion of the light may be reflected by the first sensor element(i.e., the nearest sensor element) at characteristic wavelength i(λ_(i,a)), and a second portion of the light may be reflected by thesecond sensor element (i.e., the next closest sensor element) atcharacteristic wavelength i (λ_(i,b)).

At the receiver 130, the first portion of the light will have traveled around trip distance of 2l₁. Therefore, if the refractive index of theoptical waveguide is n, the round trip delay for the first portion ofthe reflected light to reach the receiver is:

$t_{{rtd\_ i},a} = \frac{2l_{1}n}{c}$

where c is the speed of light in a vacuum. This round trip delay is thetime the receiver can expect to receive the first reflected opticalsignals from the interrogation of the reflective sensor elements atcharacteristic wavelength i. The second portion of the light will havetraveled a round trip distance of 2l₂. Therefore, the round trip delayfor the second portion of the reflected light to reach the receiver is:

$t_{{rtd\_ i},b} = {\frac{2l_{2}n}{c}.}$

This round trip delay is the time the receiver can expect to receive thesecond reflected optical signals from the interrogation of thereflective sensor elements at characteristic wavelength i.

A listening window (τ_(listen)) 1312 may be defined as the time duringwhich the tunable optical filter 114 for filtering the reflected opticalsignals is tuned to have a passband encompassing a particularcharacteristic wavelength of interest (i.e., the peak wavelength of areflective sensor element 122). The listening window 1312 may also beconsidered as encompassing a particular distance window from whichreflected signals will return to the tunable optical filter 114 with thecorrect delay in order to pass through the tunable optical filter 114,typically for all wavelengths covered by the sweep. In cases where thedistance window is short enough (or the separation great enough) suchthat only one of λ_(i,a) and λ_(i,b) is encompassed by the listeningwindow 1312, the sensor elements 122 may be separately interrogated byadjusting the tunable filter 114 to different delays (e.g., τ_(delay)_(—) _(i,a) and τ_(delay) _(—) _(i,b)), hence achieving time-divisionmultiplexing of the sensors. Multiple sensor elements 122 with differentwavelengths may be interrogated within the same listening window astheir return signals will be distributed in time at the receiver 130 dueto the swept wavelength of the source, allowing wavelength-divisionmultiplexing of sensors.

For embodiments without a delay between the source and receiverfiltering, (e.g., with a tunable filter that filters both the outgoingand returning optical signals), the sweep period (τ_(sweep)) may mostlikely be set to match or be a harmonic of the round-trip time from thesource 110 to the sensor element of interest and back to the receiver130. For an optical fiber 1301 having a refractive index n=1.5 and thefurthest reflective sensor element positioned 100 km away from thereceiver 130, τ_(sweep)=1.0 ms (=2*100 km*1.5/3.0×10⁸ m/s). Therefore,the sweep rate may be set to 1 kHz or a harmonic thereof (e.g., 2 kHz).

For other embodiments, the delay may be set to match the round-trip timefrom the source to a sensor element of interest and back to thereceiver. For some embodiments, the delay may be changed fromtime-to-time or over time in order to receive signals from differentsensor elements of interest located at various distances from the sourceand receiver.

This technique for setting the sweep rate of the tunable optical filteris similar to Fourier domain mode locking (FDML) techniques forconstructing a laser. Such techniques are disclosed in U.S. PatentApplication No. 2006/0187537 to Huber et al., entitled “Mode LockingMethods and Apparatus” and filed Jan. 20, 2006, and in R. Huber, M.Wojtkowski, and J. G. Fujimoto, “Fourier Domain Mode Locking (FDML): Anew laser operating regime and applications for optical coherencetomography,” Optics Express: Vol. 14, No. 8, 17 Apr. 2006, pp. 3225-37.In constructing an FDML laser, “a narrowband optical bandpass filter isdriven periodically with a period matched to the optical round-trip timeof the laser cavity, or a harmonic thereof.” In other words, the sweepperiod τ_(sweep) of the FDML laser is dependent on the length of thering cavity. In embodiments of the present invention, however, τ_(sweep)is dependent on the length to the sensing elements along the length ofan optical waveguide disposed down a borehole, for example.

It may also be noted that Rayleigh scattering and back-reflectionsoutside the time slot of the listening window may be filtered from thereflected optical signals. Therefore, their limitation on the opticalbudget or system range may be significantly reduced. In embodiments ofthe present invention, Rayleigh scattering may be filtered out by afactor equal to

$\frac{{\Delta\lambda}_{filter}}{{\Delta\lambda}_{sweep}\frac{\Delta \; t_{delay}}{\Delta \; t_{sweep}}}$

where Δλ_(filter) is the spectral width of the tunable optical filter114, Δλ_(sweep) is the wavelength range of the sweep over which thefilter is tuned, Δt_(delay) is the time delay between the swept sourcearriving at a particular wavelength and the receiver's tunable filterreaching the same wavelength, and Δt_(sweep) is the length of time forthe tunable filter to return to a given start position and be travelingin the same wavelength tuning direction during its normal sweepingoperation.

FIG. 14 illustrates another example of a reflective optical sensorsystem 1400 for swept-wavelength interrogation. In relation to FIG. 13A,the tunable optical filter 114 of FIG. 14 has been moved between thecirculator 1302 and the reflective optical elements 122. The tunablefilter 114 in such a configuration may most likely possess lowback-reflection for the system to be effective. Still, however, thetunable optical filter 114 may filter the optical signals reflected bythe sensor elements 122 as described above before the reflected signalsreach the receiver 130 via the circulator 1302.

Some embodiments may include an optional reference receiver 220 in aneffort to monitor the swept-wavelength optical signals being transmittedto the reflective sensor elements 122. The reference receiver 220 mayincorporate a reference element 116 as described above. The opticalsignals may be directed to the reference receiver 220 via a splitter(not shown), similar to the splitter 126 of FIG. 1B. The referencereceiver 220 may be independent from the receiver 130 or may beincorporated (at least partially) into the receiver 130.

As shown in FIGS. 15A-B, the tunable optical filter 114 may be locatedwithin the effective cavity of the light source for some embodiments. InFIG. 15A, for example, an optical gain element 1502 is disposed betweena reflecting endplate 1504 (e.g., a mirror) and the reflective sensorelements 122 forming the resonator of a laser. The tunable opticalfilter 114 may be used to both tune the wavelength of the sweep and tofilter the reflected optical signals. The reflected optical signals maybe routed to the receiver 130 and an optional reference receiver 220 viaone or more optical couplers 124.

As illustrated in FIG. 15B, the optical gain element 1502 may comprise again medium 1506 and a pump laser 1208 as described above. The receiver130 may comprise a sensor receiver 1230, also as described above. Forsome embodiments, the reflective sensor elements 122 may be disposed onmultiple optical fibers as shown. However, reflective sensor elementswith the same characteristic wavelength should be positioned atdifferent locations along the various optical fibers, as illustrated inFIG. 15B, such that reflections from these same-wavelength elementsoccur at different times and preferably do not overlap. In this manner,the reflections from these same-wavelength elements can be distinguishedfrom one another using TDM.

One way to solve this potential problem may be to change the order ofthe reflective sensor elements on the different optical fibers. Forexample, the order according to characteristic wavelength of thereflective sensor elements on one fiber could be reversed on a secondfiber, as illustrated in FIG. 16B, as long as sensor elements at themiddle of the two optical fibers having the same characteristicwavelength did not overlap. Another way to ensure sensor elements withthe same characteristic wavelength are positioned at different locationsalong the various optical fibers may be to shift the reflective sensorelements 122 on one fiber with respect to a second fiber.

The embodiment of FIG. 16A is similar to FIGS. 15A-B, but the resonatorusing the reflecting endplate 1504 has been replaced with a ring cavityusing the optical gain element 1502 and isolators 150 on either side ofthe gain element. In addition, an optical circulator 1302 directs lightemitted from the ring cavity to the tunable optical filter 114 and thereflective sensor elements 122 and directs the reflected optical signalsback into the ring cavity. Thus, the reflected light may be directedback into the ring cavity for further stimulated emission within thegain element 1502. Here again, the tunable optical filter 114 may beused to both tune the wavelength of the sweep and to filter thereflected optical signals. The reflected optical signals may be routedto the receiver 130 and an optional reference receiver 220 via one ormore optical couplers 124.

For some embodiments, as illustrated in FIG. 16B, the tunable opticalfilter 114 may be located anywhere along the ring cavity of the lightsource. Here again, the tunable optical filter 114 may be used to bothtune the wavelength of the sweep and to filter the reflected opticalsignals. Furthermore, for some embodiments, the receiver 130 may becoupled to the ring cavity via an optical coupler 124.

The operations, calculations, and timing described above with respect toFIGS. 13A-C may also apply to the embodiments of FIGS. 14-16B. In someof these embodiments using only a single tunable optical filter 114 forboth the source sweep and the reflected signals, however, a delaybetween changing the sweep wavelength and adjusting the passband of thefilter for the reflected optical signals may not be possible. In otherwords, the delay may be zero for such embodiments.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method comprising: providing light, wherein awavelength of the light is changed according to a sweep function;interrogating one or more optical elements with the wavelength-sweptlight to produce optical signals; filtering the optical signals, whereina bandpass wavelength range is changed based on the sweep function tofollow the changes in the light's wavelength; and receiving the filteredoptical signals for processing.
 2. The method of claim 1, wherein theoptical elements comprise transmissive optical elements.
 3. The methodof claim 1, wherein the sweep function comprises a constant sweep rate.4. The method of claim 3, wherein the sweep rate is set to a tripfrequency or a harmonic thereof, wherein the trip frequency is equal toa speed of light in an optical waveguide for conveying the opticalsignals divided by a trip distance of the light from a light source tothe furthest one of the optical elements and then to a receiver via theoptical waveguide.
 5. The method of claim 1, wherein filtering theoptical signals comprises filtering out Rayleigh scattering andback-reflections from an optical waveguide conveying the opticalsignals.
 6. The method of claim 1, further comprising delaying the sweepfunction before filtering, such that the changing of the bandpasswavelength range is delayed from the changing of the light's wavelength.7. The method of claim 1, wherein delaying the sweep function comprisesdelaying the sweep function by a trip distance of the light, in anoptical waveguide for conveying the optical signals, from a light sourceto one of the optical elements and then to a receiver divided by a speedof light in the optical waveguide.
 8. The method of claim 1, whereinfiltering the optical signals comprises tuning an optical bandpassfilter.
 9. The method of claim 8, wherein providing the wavelength-sweptlight comprises tuning the same optical bandpass filter.
 10. The methodof claim 1, wherein providing the wavelength-swept light comprisestuning an optical bandpass filter receiving broadband light.
 11. Anapparatus comprising: a light source for providing light and configuredto change a wavelength of the light according to a sweep function; oneor more optical elements configured to react to the wavelength-sweptlight from the light source at characteristic wavelengths producingoptical signals; a tunable bandpass filter configured to filter theoptical signals, wherein a bandpass wavelength range of the filter ischanged based on the sweep function to follow the changes in the light'swavelength; and a receiver for processing the filtered optical signals.12. The apparatus of claim 11, wherein the sweep function comprises aconstant sweep rate.
 13. The apparatus of claim 12, wherein the sweeprate is set to a trip frequency or a harmonic thereof, wherein the tripfrequency is equal to a speed of light in an optical waveguide forconveying the optical signals divided by a trip distance of the lightfrom the light source to one of the optical elements and then to thereceiver via the optical waveguide.
 14. The apparatus of claim 11,further comprising means for delaying the sweep function beforefiltering, such that the changing of the bandpass wavelength range isdelayed from the changing of the light's wavelength.
 15. The apparatusof claim 14, wherein the means for delaying the sweep function isconfigured to delay the sweep function by a trip distance of the light,in an optical waveguide for conveying the optical signals, from thelight source to one of the optical elements and then to the receiverdivided by a speed of light in the optical waveguide.
 16. The apparatusof claim 11, wherein the light source comprises a broadband light sourceemitting broadband light and a tunable optical bandpass filter forfiltering the broadband light and emitting a portion of the light withinan adjustable narrow wavelength range as the changing light'swavelength.
 17. The apparatus of claim 11, wherein the tunable filter isconfigured to change the light's wavelength according to the sweepfunction.
 18. The apparatus of claim 17, wherein the light source is alaser and the tunable filter is disposed within a cavity of the laser.19. The apparatus of claim 11, further comprising a circulator fordirecting the light to the optical elements via the tunable filter andfor directing the filtered optical signals to the receiver.
 20. Theapparatus of claim 11, further comprising a circulator for directing thelight to the optical elements and for directing the optical signals tothe tunable filter and the receiver.
 21. The apparatus of claim 11,wherein optical elements having the same characteristic wavelength arelocated at different distances, from the receiver, along one or moreoptical waveguides for conveying the optical signals.
 22. The apparatusof claim 11, wherein the light source comprises an amplifier foramplifying light emitted from an amplified spontaneous emission (ASE)source.
 23. The apparatus of claim 22, wherein the amplifier comprises adoped fiber amplifier (DFA).
 24. The apparatus of claim 11, wherein theoptical elements comprise transmissive optical elements.